GPIIb-IIIa, integrin alphaIIb-beta3, mediates platelet aggregation by serving as an inducible receptor for fibrinogen and other adhesive proteins. This function establishes an essential role for GPIIb-IIIa in hemostasis and thrombotic diseases, the leading cause of death in the U.S. Because of the central contribution of GPIIb-IIIa in physiology and pathophysiology, detailed understanding of the structure and function of this adhesion receptor is essential, and this objective is the ultimate goal of this proposal. The applicants hypothesize that an unidentified discrete amino acid sequence within GPIIb-IIIa serves as a contact site for the carboxy-terminus of the gamma chain of fibrinogen, and engagement of this site is essential for platelet aggregation. Accordingly, the gamma-chain contact site within the receptor will be localized utilizing chemical crosslinking, synthetic peptides and molecular biology approaches (Specific Aim 1). Recently, they have proposed a molecular mechanism for ligand binding to GPIIb-IIIa: recognition sequences within the ligand, contact sequences within the receptor, and divalent cations form a ternary intermediate complex; and, subsequently, cation is displaced from this complex during the ligand binding event. This model will be put to a critical test (Specific Aim 2). The cytoplasmic domain of GPIIb-IIIa forms a binding site for intercellular constituents, such as cytoskeletal elements and signaling molecules. They hypothesize that the cytoplasmic domain of each subunit exists in a specific conformation and may, in fact, interact with each other to form a unique structure. They propose to characterize the conformation and interaction of these cytoplasmic tails utilizing specifically designed and constrained synthetic peptides (Specific Aim 3). They further hypothesize that the conformation and complexation of the cytoplasmic tails of the GPIIb and GPIIIa subunits control the activation state of the receptor and mediate specific interactions with intracellular constituents. These specific functions of he cytoplasmic tails will be tested (Specific Aim 4). Overall, the proposed studies are anticipated to provide insights into the structure and function of GPIIb-IIIa, may lead to the design of more effective and safer agents to control and prevent thrombosis, and should yield broadly applicable information regarding ligand binding and activation of other members of the integrin adhesion family.